FIG. 1 is a diagram of a conventional optical telemeter.
As shown in FIG. 1 many current optoelectronic telemeters use a light emitting diode (LED) 1 disposed at a first observation point and associated with a modulator 2 controlled by a high frequency oscillator 3 operating at a frequency F.sub.T which is preferably several MHz, e.g. 15 MHz. The LED is disposed at the focus of a first lens 4 to emit an infrared light signal which is amplitude modulated at the frequency of the high frequency signal generated by the oscillator 3. The light signal is in the form of a thin beam which is thus capable of propagating a considerable distance.
A trihedral reflector 5 disposed at a second observation point returns the beam parallel to itself in a symmetical arrangement whose center is very close to the vertex of the reflector 5, and the returned beam is focused by a receive lens 6 adjacent to the above-mentioned send lens 4.
A photodiode is located at the focus of the second lens 6, and in general the photodiode will be of the avalanche type (APD). It feeds a signal to an amplifier stage 8 which delivers an electrical output signal which is modulated and phase shifted by an angle .phi. relative to the send modulation signal. The angle .phi. is related to the propagation time of the signal by the following equation: ##EQU1## in which:
F.sub.T is the frequency at which the LED 1 is modulated;
D is the distance between the prism 5 of the telemeter (1, 7) (i.e. half the total path followed by the electromagnetic radiation); and
c is the speed of light.
In order to determine the distance separating the two observation points with high accuracy (i.e. about 1 mm), it is theoretically necessary to have very fine measuring resolution (about 3.3.times.10.sup.-12 sec).
For this reason, when the modulating signal is at a high frequency, it has become conventional to measure the phase shift between two low frequency signals.
To begin with, the received modulation signal generated at the output from the amplifier stage 8 is transposed to a low frequency signal (F.sub.O -F.sub.T) (e.g. about 1,500 Hz) by mixing the received modulation signal in a mixer 9 with the signal from a local oscillator 10. It should be recalled that the low frequency signal obtained at the output from the mixer conveys the same phase information as the received high frequency modulation signal.
Secondly, the high frequency F.sub.T and the high frequency F.sub.O from the local oscillators 3 and 10 are applied to a second mixer 11 to produce a reference low frequency signal.
This low frequency reference signal is applied together with the high frequency F.sub.O signal from the oscillator 10 to a digital phasemeter 16, while the low frequency transposed received modulation signal from the mixer 9 is also applied to the phasemeter 16, via a low frequency amplifier 12 an automatic gain control stage 13. a bandpass filter 14 for the frequency band .DELTA..nu..sub.1, and a pulse shaping stage 15 which transforms the analog signal from the filter output into a logical square wave signal. These various components are designed so that phase measurement is independent of the signal amplitude of the received modulation.
The digital phasemeter 16 determines the amplitude of the phase shift .phi. between the emitted signal and the received signal. Phase samples .phi..sub.i generated by the phasemeter 16 are processed by a processor unit 17 which determines the arithmetic mean .phi..sub.A of the samples: ##EQU2##
The symbol " " is used in the present application to designated estimated values.
The processor unit 17 then determines the amplitude of the distance separating the two observation points on the basis of equation (1).
The signal produced by the processor unit 17 is digitized to display the distance on a display 18.
It should be observed that to resolve the ambiguity due to each passage of the phase angle .phi. through 2.pi., i.e. multiples of the distance c/2F.sub.T, measurements are conventionally performed at one or more frequencies whidh are lower than the frequency F.sub.T. These arrangements are well known to the person skilled in the art and are not described further.
It should also be observed that the filter 14 conventionally has a bandwidth .DELTA..nu..sub.1 which is relatively narrow (about 100 Hz) in order to reduce detection noise levels before being applied to the input of the phasemeter 16. The phase samples .phi..sub.i are thus correlated and it is not necessary to sample at a period of less than kT, where T is the low frequency period obtained at the outputs of the mixers, and k is a numter such that kT is less than 1/.DELTA..nu..sub.1.
In fact, taking the arithmetic mean .phi..sub.A constitutes a stage of digital filtering in addition to the analog filtering.
It should also be observed that with a large number of elementary measurements .phi..sub.i which are highly correlated, the variance of the result V(.phi..sub.A) tends towards a limit which does not depend on the number of samples, but rather depends only on the measurement time t.sub.m : ##EQU3## (where kO is a constant).
The Applicant seeks to provide an improved telemetry method which enables the accuracy of the measurement to be improved and also enables the range to be increased.
The Applicant has observed that the medium through which the signal propagates between the two observation points is generally formed by masses of air in movement which are not homogeneous in temperature. This results in a field of refractive index gradients n.sub.i in which the amplitudes and directions of the field are random in time.
Although the resulting optical path is in fact a sinuous path, calculation and experiment have shown that the increase in path length between the two observation points is negligible, and thus that turbulance does not directly affect the phase measurement.
In contrast, turbulance does occasionally cause the signal to disappear thereby erratically reducing the signal to noise ratio and thus reducing the accuracy of the measurement since: ##EQU4## in which:
V(.phi..sub.i) represents the variance in the phase samples .phi..sub.i ;
S represents the received signal of amplitude which is random in time;
N.sub.A represents the r.m.s. noise value; and
k.sub.1 is a constant.
The spectrum of the amplitude modulation applied to the signal by the turbulance, herein after referred to as the turbulance spectrum has a bandwidth .DELTA.F.
Further, experiments have shown that the amplitude of the signal expressed in dB is a random variable obeying the normal distribution law, i.e. it can be defined by an average .mu..sub.s and a standard deviation .sigma..sub.s.
Attempts could be made to improve the performance of a telemeter in the presence of turbulance, e.g. by using lenses of large diameter in order to simultaneously increase the average .mu..sub.s of the signal and to reduce the standard deviation .sigma..sub.s thereof.
However, such a solution also increases the volume, the weight and the price of the apparatus. Furthermore, such a solution does not necessarily improve the accuracy of the final measurement.
Preferred implementations of the present invention provide a new method of determining the distance between two points by reducing the sensitivity of a telemeter to turbulance without changing the outward appearance of the telemeter instrument.